The monarch role in the CoE is a political one rather than spiritual. Its simply symbolic to the CoE being the state church and the unity between it and the English/British state.

It's not require to be a good Protestant to hold it (I'm sure you could think of a few kings whose life weren't exactly a sign of Christian virtues and morals...) in fact in the past the head of the CoE wasn't even require to be a Protestant as it was the case with the Catholic James II that was the Supreme Governor the CoE and manage to hold the support of important churchmen (including the Archbishop of Canterbury) even after he lost the throne, a support that extended to his Catholic son and grandson (the latter briefly converted to Anglicanism).

There are in fact two titles - Defender of the Faith and Supreme Governor of the Church of England.

Although Henry VIII was originally given the title Defender of the Faith but it was later stripped from him after his break from Rome. He then had the English Parliament give it back to him.

Originally Henry also had himself declared Head of the Church of England under the first Act of Supremacy and then that was repealed under Mary so Elizabeth had to start again with another Act of Supremacy but she had to use the title Supreme Governor of the CoE.